1. Field of the Invention
The present invention relates to a synchronous motor control method and a synchronous motor control system.
2. Description of the Related Art
In a synchronous motor, induced voltage produced by speed electromotive force increases with the increase of the rotating speed of the rotor of the synchronous motor. The polarity of the induced voltage is opposite that of the output inverter voltage of an inverter circuit. Therefore, increase in the induced voltage causes the output torque of the synchronous motor to decrease. If the voltage across the terminals of the synchronous motor drops instantly, the output torque of the synchronous motor decreases furthermore.
Therefore, when high rotating speed and high torque are necessary, a high-speed synchronous motor must be employed instead of a low-speed synchronous motor. The allowable maximum current and allowable maximum voltage of the high-speed synchronous motor are high, the high-speed synchronous motor is inevitably large and the manufacturing cost of the high-speed synchronous motor is high.
There is a mounting demand for space reduction and cost reduction in recent years, then, efforts have been made in recent years to make a low-speed synchronous motor produce a high torque when the rotating speed is high. From the point of view of miniaturization and improvement of efficiency, suppression of heat generation during operation and power factor improvement are desired.
First and second prior control methods of controlling a synchronous motor to improve the speed-torque characteristic of the synchronous motor are disclosed in Japanese Patent Laid-Open Publication (JP-A) 11-18498 and Japanese Patent Laid-Open Publication (JP-A) 9-84400. The first control method disclosed in JP-A 11-18498 makes a predetermined d-axis current flow during high-speed operation. The first control method increases the d-axis current according to a value obtained by multiplying the rotating speed by a predetermined constant. The second control method disclosed in JP-A 9-84400 supplies a d-axis current only during high-speed operation and increases the d-axis current according to the rotating speed of the rotor. The second control method starts supplying the d-axis current at a first set speed near a speed at which voltage saturates, increases the d-axis current according to a linear increasing function and fixes the d-axis current at a fixed value upon the increase of the rotating speed to a second predetermined speed. The first and the second control method control the d-axis current on the basis of the rotating speed to suppress the reduction of the output torque due to the increase of the induced voltage with the increase of the rotating speed.
The first and the second control methods only control the d-axis current on the basis of the rotating speed. Consequently, a d-current corresponding to a rotating speed flows even in a state where a low torque is necessary. The prior control methods supplies the d-axis current regardless of a necessary torque. Therefore, it is very probable that an excessively high current is supplied to the synchronous motor. Consequently, the improvement of the efficiency of the synchronous motor is obstructed, the synchronous motor generates heat at a high rate and the power factor decreases.